In the semiconductor industry, robot arms have typically been employed to move semiconductor wafers from one location to another. It is desired in the design of a robot arm in the semiconductor industry to have the end effector maintain the same orientation when the arm moves radially inward and outward. As discussed, for example, in U.S. Pat. Nos. 4,299,533 and 5,064,340, robot arms have been provided with two links and an end effector mounted at a distal end of the second link. The proximal end of the first link is mounted coaxially with a pulley, referred to herein as the "fixed" pulley. The proximal end of the second link is pivotally connected to the distal end of the first link. A first belt connects the fixed pulley to a second link pulley at the proximal end of the second-link, whereas a second belt connects a housing fixed to the distal end of the first link to a pulley on the end effector. If the lengths of the links are equal, and if the pulleys are selected so that the ratio of rotation of the end effective to rotation of the second link is -1/2 and the ratio of rotation of the second link to rotation of the first link is -2, the end effector will remaining fixed orientation but will move radially when the first link rotates relative to the fixed pulley.
While this approach is effective, it imposes some serious design constraints. To provide the desired ratios, a relatively large end effector pulley is needed. For the end effector to rotate one-half the rotation of the second link, the pulley at the end effector must be twice as large as the housing on the distal end of the first link. When the pulley at the end effector is large, it adds significant mass to the end of the robot arm, increasing inertia and making it much more difficult to control precisely the movements of the arm.
Also, to provide the desired ratios, the second link pulley and the housing at the distal end of the second link must be of relatively small diameter, i.e., half the diameter of the fixed pulley. This makes it difficult to connect to wires, pneumatic tubes, or other elongated components extending from the first to the second neighboring link through a hollow bore at the exit of the second link pulley and housing. Such wires, tubes and other elongated components are usually part of the control system for controlling the end effector. Thus, it would be desired to create a robot arm with enough space at the joint between the links so that these wires and tubes can pass through the center of the joint.
The robot arm of the present invention addresses the problems set forth above and is described more fully below.